For the purpose of improving light transmittance, an antireflection film is provided on a surface of an optical component constituting an optical device.
Reflectance becomes theoretically zero by coating with a low-refractive-index material to an optical thickness of λ/4 based to wavelength λ, the low-refractive-index material having a refractive index (nc) represented by the following relational expression to refractive index ng of a substrate in air:nc=√ng  (Expression 1)
As an antireflection film for an optical element which requires a lower-reflection effect, instead of the above-described single layer, a multilayer film is used, in which a high-refractive-index film and a low-refractive-index film are alternately laminated. In this case, a low-refractive-index material is important for the uppermost layer on the air side. On the other hand, low-refractive-index films are widely attempted to be made porous in order to decrease the refractive indexes.
As a method for forming porous films, a wet process, not a dry process such as vacuum deposition, is effective. The wet process has the advantage of easy formation of the porous films because the films are formed by a coating method after coating materials are dissolved or dispersed in solvents.
PTL 1 and NPL 1 each disclose a method for forming magnesium fluoride by thermal disproportional reaction. That is, PTL 1 and NPL 1 each disclose a method for forming magnesium fluoride by applying a fluorine-containing magnesium compound or a magnesium fluorocarboxylate compound used as a precursor to a substrate and then performing thermal disproportional reaction.
Such compounds and fluorocarboxylic acids generally have high hygroscopicity and thus produce defects in coating films due to wetting when coating solutions containing a mixture of a magnesium compound and a trifluoroacetic acid are applied, thereby causing the problem that uniform coating films cannot be obtained.
PTL 2 or PTL 3 discloses a method for stably forming a coating solution by adding a stabilizer to a magnesium compound. However, this literature does not describe a countermeasure against the occurrence of film defects when a coating solution is applied.